Charles Kupperman, a former deputy to national security adviser John Bolton, declined to appear on Capitol Hill Monday for a deposition with House impeachment investigators, and instead sought a federal judge's ruling on whether to comply with the congressional subpoena over the White House's objections.
Democrats say Kupperman was on the July 25 phone call between President Donald Trump and Ukrainian President Volodymr Zelenskiy that is at the focus of the impeachment investigation.
The standoff over his appearance could complicate Democrats' efforts to obtain testimony from Bolton and other key figures who were in the White House and around President Trump during events at the center of the Ukraine impeachment inquiry.
(MORE: Diplomat who raised concerns about Ukraine aid in texts testifies; Democrat says it could 'accelerate' impeachment timeline)"Dr. Kupperman cannot satisfy the competing and irreconcilable demands of both the Legislative and Executive Branches, and there is no controlling judicial authority definitively establishing which Branch’s command should prevail," Charles Cooper, an attorney for Kupperman who also represents Bolton, said in a statement. "Under our system of Government, Constitutional disputes between the Legislative and Executive Branches should be adjudicated by the Judicial Branch, not by private citizens like Dr. Kupperman."
Kupperman filed a lawsuit Friday asking a federal judge whether he should comply with the subpoena from the House committees conducting the impeachment inquiry.
House Intelligence Committee Chairman Adam Schiff, D-Calif., criticized Kupperman's decision not to come forward, noting that the committees have already heard from nine witnesses over the last few weeks, despite concerns raised by the White House.
(MORE: Ambassador to EU Sondland told Congress quid pro quo described by Mulvaney would be improper)"A private citizen cannot sue the Congress to try to avoid coming in when they're served with a lawful subpoena," he said. "We expect that the court will make short shrift of that argument, but nonetheless, we'll move forward."
Schiff said Democrats are "are not going to allow the White House to engage us in a lengthy game of rope-a-dope in the courts," and that they would use the efforts to block testimony to inform their impeachment recommendations. Kupperman's failure to appear on Capitol Hill Monday "may warrant a contempt proceeding," he added.
"I think we can infer from the White House opposition to Dr. Kupperman's testimony that they believe that his testimony would be incriminating of the president," Schiff said. "If this witness had something to say that would be helpful to the White House, they would want him to come and testify, they plainly don't."
Gordon Sondland, the U.S. ambassador to the European Union, was also spotted on Capitol Hill Monday as he reviewed a transcript of his recent deposition with impeachment investigators, according to a source familiar with his appearance.
Democrats have scheduled five additional witness interviews in their impeachment inquiry this week, including a Thursday deposition with Timothy Morrison, a National Security Council official who featured prominently in the testimony of Ambassador William Taylor, the top U.S. diplomat in Ukraine who appeared on Capitol Hill last week.
Morrison plans to appear on Thursday if subpoenaed by lawmakers, which would make him the first current White House official to testify in the impeachment probe.
“If subpoenaed, Mr. Morrison plans to appear for his deposition. We will not be commenting on what he will say to the Committees," his lawyer, Barbara Van Gelder, told ABC News.
Republicans and the White House continue to criticize Democrats' investigation, and have raised questions about the closed-door proceedings and accused the majority of limiting their access to transcripts and materials gathered in the probe.
Democrats say they are planning a series of public hearings in the impeachment investigation, which could start as early as next month, according to Democratic sources.